Symmetry-Dependent Optical Properties of Stellated Nanocrystals

Stellated metal nanostructures are a class of plasmonic colloids in which large electric field enhancements can occur at their sharp tips, making them excellent candidates for surface-enhanced Raman spectroscopy (SERS) and surface-enhanced infrared spectroscopy (SE-IRS) platforms. Shape-dependent rules for convex polyhedra such as cubes, nanorods, or octahedra exist, which describe far-field scattering and near-field enhancements. However, such rules are lacking for their concave counterparts, which include stellated structures. Here, the optical response of stellated Au nanocrystals with O h, D 4 h, D 3 h, C 2 v, and T d symmetry were modeled to systematically investigate the role of symmetry, branching, and particle orientation with respect to excitation source using finite difference time domain (FDTD) calculations. Furthermore, these results are compared to experimentally obtained localized surface plasmon bands determined by UV–visible (UV–vis) spectroscopy for ensembles of similarly shaped metallic nanostructures. These UV–vis spectra include that of the first reported all-Au octopodal structure achieved from seed-mediated methods. From these studies, symmetry-dependent rules governing the far-field and near-field optical response are outlined. These rules can be applied to a variety of concave structures to yield the optimized far-field and near-field responses for SERS and SE-IRS platforms.